The present invention relates to a self-adjusting braking device for units for feeding weft to textile machines in general and particularly to weaving looms of the gripper or bullet type.
More specifically, the invention relates to self-adjusting braking devices of the type disclosed in EP-0 536 088 and in Italian patent 1,259,567, referenced hereinafter briefly as conventional devices.
Weft feeders are devices used in weaving processes and typically comprise a fixed cylindrical drum on which an arm that rotates in a fishing-reel fashion winds a plurality of turns of thread that constitute a weft reserve, means for producing the advantage of the turns from the base toward the end of the drum, and braking means for braking at the exit the thread that unwinds from the drum when requested by the loom or the like at each weft insertion and impart thereto the correct mechanical tension, which is indispensable for the correct unwinding of the thread.
The expression xe2x80x9cself-adjusting braking devicesxe2x80x9d is used to designate, in the present description, braking means capable of automatically varying the braking action applied to the thread that unwinds from the drum of the feeder when the advancement speed of the thread varies, in order to keep its mechanical tension substantially constant.
For this purpose, the conventional self-adjusting braking devices are constituted by a substantially frustum-shaped braking body, which is suspended elastically in front of the fixed drum of the feeder, to which it is tangent at an exit circumference that is slightly smaller than the maximum circumference of the drum; the drum, in order to facilitate the unwinding of the thread, has a rounded exit rim whose cylindrical surface is blended with the flat front one.
The thread advances between the drum and the frustum-shaped braking body, onto which it discharges the axial component of its mechanical tension. When the tension rises, as the advancement speed of the thread increases during weft insertion, the axial component tends to displace the braking body in contrast with the elastic action of its suspension means and causes, or tends to cause, its separation from the drum, with a consequent and corresponding decrease in the braking action, which in this manner adjusts itself according to the advancement speed of the thread, keeping its respective mechanical tension substantially unchanged, as mentioned.
For correct operation in the specified sense, the frustum-shaped braking body of the self-adjusting braking devices must have certain characteristics and mainly: considerable radial elasticity, substantial axial rigidity, and limited inertia. For this purpose, it is known to provide the frustum-shaped body by means of a fabric impregnated with resins or with a laminate, providing it internally with a metallic cladding that has a mainly wear-resistant function. The cladding usually covers and protects a limited band of the frustum-shaped body which straddles the exit circumference of the drum of the feeder, and is constituted by a steel lamina of modest thickness, for example between 0.5 and 5 tenths of a millimeter.
It is also known to improve the response of the braking body to variations in thread tension by providing it with a metallic ring that has a raised rim, is arranged at the smaller or end cross-section of the body and is adapted to divert the path of the thread so that the thread discharges onto the braking body the axial component of its tension not only at the region of tangential contact with the drum but also at the end region of the braking body; the arrangement being such as to improve significantly the overall elastic response of the braking device.
However, in some cases, for example in the presence of lower-count threads, such as wool yarns for combed fabrics, the radial elastic yielding of the conventional braking bodies can be insufficient and accordingly the self-adjusting action of the braking applied to the thread by such a braking body can be entirely unsatisfactory. In particular, in conventional braking bodies the presence of the metallic cladding lamina limits significantly, in the very region of contact with the drum, the elastic deformability of the body in a radial direction, since the lamina, which is usually applied and glued directly to the inner face of the body, increases its thickness at the expense of its flexibility. On the other hand, a generalized reduction in the thickness of the material of the frustum-shaped braking body cannot exceed certain limits, on penalty of a consequent unacceptable deterioration of the axial rigidity of the body.
In an attempt to obviate this drawback, it has already been proposed to replace the rigid frustum-shaped body with a metallic body formed by a plurality of independent tongues arranged along the generatrices of a frustum-shaped solid and connected to each other in the region of the maximum circumference of the solid, by means of a band having a continuous surface, which engages, with an elastic tangent contact, the exit portion of the drum of the weft feeder in order to brake the thread, and in which the tongues are free with respect to each other at the region of the solid that has a smaller circumference and where the tongues are partially coupled to a cup for supporting the braking body.
An arrangement of this type, disclosed in WO 94/10075 and WO 94/12420, significantly improves the radial elastic deformability of the braking body but sacrifices almost completely its axial rigidity, compromising unacceptably the self-adjusting operation of the braking system.
According to further conventional embodiments, the portion of the frustum-shaped braking body that is in contact with the drum of the feeder is rendered elastically flexible in a radial direction through the adoption of a spring-loading means interposed between said body and said drum. Embodiments of this type are disclosed in WO 99/20557, wherein such interposed means is constituted by a metallic annular element comprising a plurality of elastically flexible tongues which are connected one another in the region of the braking body that has a larger circumference but are mutually free in the region having a smaller circumference, and in WO 00/20316, wherein said interposed means is constituted by a crown-like disk with a channel-shaped cross-section and provided with a plurality of radial slits.
The main drawback of these further embodiments is the structural complexity of the braking body and essentially the increased dimensions and mass of the body, whose inertia reaches values that are also unacceptable for correct and efficient self-adjusting operation of the braking system.
The aim of the present invention, starting from the notion of such drawbacks of conventional self-adjusting braking devices, is essentially to eliminate them.
Within this aim, an object of the present invention is to provide a self-adjusting braking device with a braking body that is extremely efficient, has significantly improved characteristics of radial elasticity and axial rigidity and at the same time has a very low inertia, so as to be significantly more effective in terms of the self-adjusting behavior of the braking system. In particular, with the device according to the invention the self-adjusting behavior of the system is enhanced significantly, in that the device needs very low thread tensions in order to modulate the braking action effectively and is thus adapted to operate with any type of thread, especially with lower-count and/or combed threads and even with very high weft insertion speeds, which are typical of modern weaving looms.
Another object of the present invention is to provide a self-adjusting braking device with a frustum-shaped braking body that is structurally very simple, reliable in operation, and economically advantageous.
Another object of the present invention is to provide a self-adjusting braking device with a braking body that has minimal dimensions and requires no stiffening of the respective elastic suspension structure and indeed allows, thanks to its significantly reduced mass, to reduce the elastic rigidity of said structure, further enhancing the sensitivity of the braking system.
According to the present invention, this aim and these and other objects that will become better apparent from the following detailed description are achieved with a self-adjusting braking device having the specific characteristics defined in the appended claims.
Substantially, the invention is based on the inventive concept of providing a braking device with a frustum-shaped braking body that is suspended elastically in front of the drum of the feeder and is pushed into elastic tangent contact with said drum, wherein said braking body is characterized at least by the presence, at a larger-diameter region thereof meant to make contact with said drum, of a discontinuous annular contact element which comprises a plurality of through slits which are arranged so as to be mutually equidistant and delimit a row of contact laminas, each of which has both ends joined to the braking body and monolithic therewith; the annular contact element, arranged at a region of contact between the braking body and the drum, being arranged symmetrically on either side of an exit circumference of said drum.
According to another characteristic of the invention, the slits of the annular contact element are inclined, with respect to the generatrices of the frustum-shaped braking body, so as to be orientated with an angle alpha, preferably a right angle and in any case an angle between 65 and 115xc2x0, with respect to the path traced by the unwinding thread directly ahead of the exit circumference of the drum of the feeder. According to the invention, furthermore, the material of the frustum-shaped braking body is chosen from the following group of materials: elastic steel, thermoplastic polymeric material, thermosetting polymeric material, engineering polymer reinforced with carbon fibers, carbon fiber or glass fiber fabrics; and said body can have, in the part below the annular contact element, lightening holes and/or cutouts which are adapted to reduce significantly the mass and therefore the inertia of the braking body.